Most conventional coating resins are insoluble in water. Therefore, in general practice they have been dissolved in a suitable organic solvent or dispersed in water with the aid of an emulsifying agent or surfactant in order to provide a coating composition suitable for application to a substrate surface. A serious disadvantage of organic solvent solutions is that they are potentially toxic, flammable, and environmental pollutants.
Water reducible coatings do not present these problems and are, therefore, highly desirable. For this reason various water reducible coating resins, such as the one described in U.S. Pat. No. 4,474,926, have been developed. Water reducible coatings which utilize such resins have been developed for a variety of purposes and have been widely accepted in many applications such as highway striping paint.
There is a need for a coating for metal substrates which provide improved corrosion and rust resistance. Such coatings could be, for example, beneficially utilized in the automotive industry. It would be particularly desirable for such coatings to be of the water reducible type.
For purposes of this patent application, an aqueous coating system is considered to be a colloidal dispersion of a resin in water which can be reduced by the addition of water and which forms a durable coating when applied to a substrate surface. The term aqueous coating system is used herein interchangeably with the term water reducible coating. Other names which are sometimes applied to water reducible coatings are water born, water solubilized, and water dilutable.
U.S. Pat. No. 4,968,741, discloses a process for producing a neutralized latex that is useful in the manufacture of water reducible coatings which comprises:
(1) free radical aqueous emulsion polymerizing at a pH of less than about 3.5 a monomer mixture which comprises, based on 100 weight percent monomers: (a) from about 45 to about 85 weight percent vinyl aromatic monomers, (b) from about 15 to about 50 weight percent of at least one alkyl acrylate monomer, and (c) from about 1 to about 6 weight percent of at least one unsaturated carbonyl compound; in the presence of about 0.5 to 4.0 phm at least one phosphate ester surfactant and in the presence of about 0.5 to 4.0 phm of at least one water insoluble nonionic surface active agent to produce a latex; and
(b 2) neutralizing the latex with ammonia to a pH which is within the range of about 7 to about 10.5 to produce the neutralized latex.
The latices made utilizing the technique of U.S. Pat. No. 4,968,741 are outstanding for utilization in the formulation of coatings having improved moisture vapor transmission resistance and water spotting resistance. When applied to metal substrates, coating formulations made with such latices provide excellent corrosion and rust resistance. The latices described in U.S. Pat. No. 4,968,741 have proven to be extremely difficult to synthesize in large scale commercial operations. Attempts to synthesize such latices in large polymerization vessels, such as 500 gallon reactors, has resulted in severe coagulum formation.